Traditionally a seat belt retractor has a generally cylindrical spool on which seat belt webbing is wound. The spool is rotatably mounted to wind-up or pay out seat belt webbing, depending upon the direction of rotation. The spool is biased in the wind-up direction by a clock spring so that the seat belt maintains a relatively low level force on a vehicle occupant tending to pull him back into the vehicle seat in a comfortable manner, but allowing him to move forward to adjust a car radio and so forth. An acceleration sensor detects acceleration or deceleration above a predetermined level, as an indication of a crash, and locks the retractor against the further payout of webbing to securely restrain the vehicle occupant in the vehicle seat.
Modern seat belt systems include a load limiting function that allows some controlled payout of the belt webbing before locking fully, so as to more gradually decrease the momentum of the vehicle occupant. This is often achieved using a torsion bar mounted coaxially in the spool. The torsion bar twists about its own axis, after the spool mechanism is locked and typically allows up to two turns of the spool. In this way the crash forces felt by the vehicle occupant increase linearly as the crash forces increase, until the torsion bar begins to be twisted. The torsion bar stops twisting when the load applied from the vehicle occupant diminishes. The locking wheel is locked in position by the lockbar, but the spool can turn only when the torsion bar torsion bar is twisted allowing the spool to turn.
In an improved load-limiting seat belt retractor described in co-pending U.S. application Ser. No. 10/241,489, a second stage of operation is provided by a wire element housed in the spool body. The wire element works in parallel with the torsion bar to create a higher level of load limiting for a predetermined time or distance. Thus when the crash sensor first senses a crash a first stage of operation involves both the torsion bar twisting and the wire being either bent or twisted or pulled out of its housing in the spool to absorb the crash forces. Then the wire is fully bent, twisted or extended, depending on the mechanism employed. Then the crash forces are applied solely to the torsion bar and a second stage of operation involves only the torsion bar. This second stage with its lower level of load limiting works in conjunction with an airbag to share the load.
However, a disadvantage arises if a second impact occurs because the airbag has deployed due to the first impact and is no longer effective to share the restraining load with the seat belt retractor. Hence it is desirable for the seat belt retractor to revert to working as a standard non load-limiting retractor in the case of a second impact.
The present invention can provide the advantage of improving the crash force curve further and effectively restraining the vehicle occupant during a second impact.